Circuit for delayed automatic amplification control



P 1950 H. J. VAN BEEK 2,521,493

CIRCUIT FOR DELAYED AUTOMATIC AMPLIFICATION CONTROL Filed May 4, 1946 A HERMAN/5 JOJEPHUJ ZAWBHZK IN V EN TOR.

BYW

AGHVZ Patented Sept. 5, 1950 CIRCUIT FOR DELAYED AUTOMATIC AMPLIFICATION CONTROL 7 Hermanus-Josephus van Beek, EindhovemNetherlands, assignor, by mesne assignments, to Hartford National Bank. and.Trust Company, Hartford," Conn., as trustee Application May 4, 1946, Serial No. 667,247 In theNetherlands August 21, 1941 Section 1, Public Law 690, August 8,1946 Patent expires August 21, 1961 Claims. 1 T'Ehe'present invention relates 'tQautQ-matic amplification. control andmore particularly to delayed automatic amplification control for radio receivers.

The use of' anautomatic amplification control in radio-receiversrhas: a dual'object, first compensating variations intensity; brought about during therreceptiona by:fading" eifects; and sec- 0nd.1maintaining.constant: the output energy of the: receiver in a manner unchanging: with the field intensity of 1 the various: transmitters.

During the. receptionof very weak signals there is. no-neediof a control'having the: second characteristic mentioned" above vvhilaumoreover, it is generally undesirable .to further" attenuate the signals which are already tweak by compensation-as mentioned above. In modern-receivers therefore use isgenerallymaderof. a so-called delayedwautomatic volume. control 'in. which. the

amplification ofthercontrolled tubes is substantial-ly constant SO lOIlg :as the; signal intensity liesiz-below adefinite thresholdi'value, whereas :at asignal intensity which surpasses this: threshold value the amplification :of thejcontrolled amplifying tubes-is-decreasedeas the: signalilintensity a. control is not desirable; a region of very high signal intensities (originating-from' proximate transmitters) in' which substantially no fading occurs, .'=so:that-only the second object of: the automatic amplification control-which. is. mentioned above plays apart. There also occurs in intermediate-region of .mean signal intensity in which the compensation of fading effects is the main object of theautomatic" amplification control; :In the region of :high signal intensities a greatlyweffective volume control is desirable for maintaining the output energy atdifierent values of: thesignal-intensity approximately constant inadefinite position of the hand volume-control :device whereas'in the regionf t e mean signal intensity such a greatly effective volumecontrol is not desirable in View oft-he so-cal-led-selective fading. When selectiveafading occurs .and'the frequency for which-theifading occursxvaries with time, the. fading frequently. occurs only for the frequency of the carrier-wave. of. a; received signal, whereas the side-bands remain :unattenuated. If, now, due. to the .decrease of the carrier-wave. amplitude; the .amplificationof the controlled tubes :were brought. at. maximum value, then the amplitudes of. theside-bands would be excessively increased, with: the. result that an inadmissible overload of the receiver would occur.

In view thereof :it has .:previously been proposed to make the automatic volumercontrol less effective so. that the difiiculty'which occurs-with the selective fadingis obviated,:but then the secondobject of the controlmentionedabove is thus attained muchless satisfactorily.

:The:.: principle: object :of c the: present: invention is. to provide: a circuit. in-.which the above-mentioned drawbacks whichzmay' occur with selective fading are avoided, While also a substan tially constant output energy at high-signal intensity is obtained.

Further objects of the-:invention will appear from the following-description.

According. to the invention, to this enda control voltage variable Withthe intensity of-the signal to be transmitted, jointly with a substantially constant positive bias, is supplied to a control-telectrode of one-or more of the tubes to be controlled, while the'cathode lead of each of the tubes concerned-includes anadditional resistance of such value. that the said positive bias'at low signal'intensity. iscompensated by the voltage drop in the resistance.

In. order that the inventionmaybe more clear- 1y understood and readily: carried into effect, it will: be described more fully byreference to the accompanying drawing in which:

Fig. I shows a control voltagesignal voltag characteristic, r

Fig.i'2 shows a control .amplifier circuit arrangement according to. theinvention,

Fig. 3shows curves of amplifier output ener y as .a function of signal voltage, and

Fig.4 shows a suitable circuit arrangement for obtaining a. control voltage.

Fig.2 shows a part of a radio-receiver in which use is made of the invention. The oscillations to beamplified are supplied to a control grid 3 of anamplifying tube I through a condenser 5, which oscillations may originate, for example, romxa preceding.intermediateefrequency amplifier. The anode 4 of the tube 1 is connected, via an output circuit 6, to a source I of anode voltage whose negative terminal is earthed. The conductor which connects the cathode 2 to earth includes in the usual manner a resistance 8 which generates the required maximum gridbias. In series with this resistance is included a second resistance 9, the operation of which will be explained later. The two resistances 8 and 9, which may alternatively be replaced by a single resistance having a value equal to the sum of the resistances 8 and 9, are bridgedby a condenser ID. A device II supplies a control voltage which is variable with signal intensity in a manner shown by the curve of Fig. 1. The positive terminal of this device is earthed and the negative terminal is connected to the control grid 3 via a source of supply I2 and a resistance l3, the positive terminal or" the source of supply i2 being located on the side of the control grid 3.

The operation of this device is as follows: Itis assumed that the characteristic curve of tube l is such that the mutual conductance decreases when the negative grid-bias increases. When the signal intensity has a value which is located between and A, the device ll does not supply a control voltage so that the grid 3 acquires a positive voltage with respect to earth, which voltage is equal to the voltage supplied by the source 12. In the cathode circuit the resistances 8 and 9 are traversed by the cathode current, the value of the resistance 9 being such that the voltage drop which occurs across this resistance is exactly equal to the voltage supplied by the source [2. The grid 3 will consequently assume a negative voltage with respect to the cathode, of which the value is determined by the voltage drop across the resistance 8. The latter has such a value that under these conditions the tube operates on the steepest part'of the characteristic curve and consequently gives maximum amplification.

However, as soon as the signal intensity surpasses the value A, the device H supplies a control voltage which renders the grid 3 more negative with respect to the cathode 2. This results in a decrease of the cathode current and hence a decrease of the voltage drop across the re- 'sistance 9, so that the voltage supplied by the source I2 is compensated to a less extent and the negative grid-bias consequently increases to a less extent than would have been the case due to the device ll alone. Consequently, the automatic amplification control does not become fully active. When the signal intensity further increases and tube l is consequently adjusted more and more in the curved part of the characteristic curve, the cathode current decreases more and more with an increasing negative grid-bias, so that the control voltage supplied by the device ll becomes more and more eifective. At last, a substantially constant value of the cathode current is attained so that the control voltage supplied by the device I I becomes fully active. The foregoing is shown graphically in Fig. 3.

In this figure, the curve I shows the variation of the output energy measured in dbs. of an amplifier having delayed automatic amplification control as a function of the input voltage Es, if the control voltage varies as shown in Fig. 1, use being made of a usual circuit comprising one controlled tube of the type ECI-I21. The curve II shows the variation of the output energy in dbs. if use is made .of a circuit according to th nvention. It appears from the variation of the two curves that, if the input voltage from O to 'A increases by 40 microvolts, a control does not take place. The curve I further'showsthat for amplitudes of Es larger than A the control almost immediately becomes fully active. When a station is received the carrier-wave amplitude of which surpasses the threshold value A and has a value corresponding to C (1000 microvolts), the output energy will amount to 32 dbs. If the carrier-wave voltage falls below the value A, for example due to selective fading, whereas the intensity of the side-bands remains unvaried, the amplification will become maximum again so that the output energy of the side-bands is determined by the slope of the line PQ and will increase by about 24 dbs.

If, however, use is made of a circuit according to the invention the process is as follows: For an increase or Es of 10,000 microvolts from A to B, a certain control takes place. In this example, with an increase of the input voltage from 40 to 10,000 microvolts, that is to say of about 68 dbs., the output energy increases from 28 to 3'7 dbs., that is by 9 dbs. For signal intensities higher than B the control is very effective and with an increasing input voltage the output voltage remains substantially constant. In practice such a variation of the control characteristic curve becomes manifest as follows. Low signals are amplified with maximum intensity, high signals give an output voltage which is almost constant with signal intensity, whereas for the region of moderately strong signals, which is located between A and B, large variations in signal intensity give a comparatively small variation in the output voltage. When a station is received of which the carrier-wave amplitude has a value such as C, the output energy will be equal to a value of 35 dbs. If, for example due to selective fading, the carrier-wave amplitude falls below the value A, whereas the side-bands remain at normal intensity, in this case a decrease of the amplification will not occur, but now the intensity of the side-bands will increase by a little less than 21 dbs., that is more than 3 dbs. less than in the previous case. In other words the side-bands give only half of the output energy which would occur in the known circuit. From this it appears that the invention provides an important improvement.

The additional source of supply, which is indicated by 12 in Fig. 2, may be obtained in a very simple manner if for the obtainment of the control voltage use is made of the circuit described in detail below in which an impedance is connected in parallel with a control rectifier which has supplied to it the oscillations to be transmitted, said impedance'comprising the series connection of at least a smoothing resistance and a second rectifier, the second rectifier being also included in a circuit which comprises a source of supply and a second resistance. The operation of such a device has been described in detail in Dutch patent application No. 82,630 corresponding to U. S. patent application Serial No. 195,009, filed March 10, 1938, now U. S. Patent 2,200,049 issued to Carel Jan van Loon on May 7, 1940. In this case the control voltage, jointly with the constant positive direct voltage, may be derived between the cathode of the second rectifier and a tapping of the second resistance.

Fig. 4 shows a constructional example thereof. One of the anodes 20 of a duo-diode 2! has the Sign V ge to be transmitted supplied to it 5; nae-condenser 2'2. This :voltage maworiginate; for :example, from lithe: intermediateeirequency amplifienrof asuperheterodyne: receiver. .The rec-- tiryingnactionl :of' the :dioldet201 produces- .acrossaa. resistance 234a fdirectuvcltagea-wvhich is.:..-propor tionaltd the amplitude ofithealternating-voltage sup-plied: tor-the anode-:25 iThesecondxanede-tza connected: onzthe: one hand :via aresistance :25:

to: the-positives terminal: of awsorurce: of supply 126: and'on. the-other hand viassmoothing. resistance 21: to the-.anodetZii. 1 So. long as ithe alternating voltages set up at thesanodelilv are small'pt-he negative. voltage iacrossthe resistance -23:;.-whiclr is supplied: tolthe anode -Zkacrosethe:resistance. 2T, will: besmaller' than the: positive voltageoriginatingzfrom: the source 26'. Conse uentlwthe diode Z his; conductiveand'xpoint M is. placed.::ap:proximatelyl at earth potential. .If the: amplitude ofthersi-genala voltage :supplie'd to. the-anode 2th increases; the anodei2fi' becomes negative -with respectatothe cathodeata definite threshold value. The: resistance. oftthe diode space 24 isthe high arrdapointilVl? may: have taken from it-a negative voltage whichis: proportioned to the signal-voltage supplied to :the anode '16. The 1 control voltage tlsilus' obtained exhibits i the variation which is shown Icy-the curve of Fig. 1.

:If; the control; voltage; isnot derived frompoint M,; but. frorma: suitablychosen tapping K N- of re' sistancerZ-E; then: substantially I the same-control= voltage is obtained', a definite positivedirect -voltageasubstantially invariable-with the input alternating voltage being added, however, so=that' this control voltage may be directly connected to the extremityrremote. from the: grid -3, of the resistance [3 in Fig. 2.

In a receiver comprising a plurality of controlled tubes, as is'usually' the case in'superheterodyne receivers, the desired control characteristic curve mayfrequently be obtained by applying the circuit according to the invention to one--only of the controlledtubes. It is thenpreterable-that'thehi h-irequency and the intermediate-frequency amplifying tubes should be controlled the usualmanner, that is to say',-'-for examplattosupply to theirgri'ds a controtv-oltage which is taken oif from point M in Fig. 4, and to control only the mixing tube by means of a circuit according to the invention by supplying to the grid thereof a control voltage which is taken off from point N. The possibility of cross-modulation is thus greatly reduced, since, due to the form of the characteristic curve which is required in view of the obtainment of a high mutual conductance, the mixing tube often produces crossmodulation. If the desired variation of the control characteristic curve may be obtained by applying the invention to only one of the controlled stages it is consequently desirable to use therefor the mixing tube, since this tube is then controlled over a considerable portion of the control region to a less high extent than would be the case otherwise.

The various circuit elements are preferably so chosen that the value of the signal intensity at which the intensity control becomes fully active (point B of Fig. 3) at least substantially corresponds to that signal intensity above which fading phenomena are no longer to be expected.

In a practical construction of a superheterodyne receiver in which the circuit according to the invention was used only forthe control of the amplification of the hexode part of the mixing tube (type ECI-I 21), the resistances 8 and 9 together had a value of 1000 ohms, the resistance 23 a value of 018 megonm; the resi'stancefl a. value /20, such manner that the resistance between M and N amounted to. 0.5 megohm. :Thersource 2-6 supplied avoltage of 200 volts. By giving-the resistaree"?l5 and the voltage of the source. 216a.

high value, due towhich the resistance 25 maybe tapped at /20 of its value, only 1/20 of the control-- voltage is lost. If desired, the variation of thecontrol-voltage characteristic :curve :maynbe slightly influenced byusing for the source [Evin Fig.2 is'a-source of-su-pply whichis not fully-eonstantfibut a-source whose voltage is to av certain extent variable with-the signal intensity. The source ofsupply -of this kind. which is=used. .:is, for example,'the'screen-grid voltage of onejaof the controlled-tubes. 'If the screeneg-ridzis-fed via a seriesresistance, the screon-grid;voltageincreases -W-ith an increasing control voltage -since the-screen grid current decreases. sBy using :a source t2, Whose-voltageincreases withsan in creasing eontrol voltagethe action of thecontrol. voltage will initially be. delayed: still further so that therariation' ofi thepart Ofi'GlllVeiII' of Fig.3. which:isliocatettbetween the; points corresponding to-the values-A and 1B: ot thesignalavoltage E5: will initially? hav-esasteepen torm.

invention may alsoxadsvantageously be. used in. now-frequency amplifiers having automatic contrast. compression; such; asis used; inthe: recording: of: soundyongramophonez records and: in radidtransmitters.

'%The- -variations.; indntensity whichlmay be recorded on alzgramophonerecord: or rare ,permis- 'nrusicalnperformance which. may amount; to

some 'tuedbs, :so;.thatl ll'h.l50it3" supplied by the microphone: mustkbe ;.compressed,1- which may be eitectedxeither bywhandnor bywmeans'of asecalle'dl compressionramplifier. In this case itis advisable.thattherweakest passages should-comparatively 1' be amplified: mostsand I the intermediate region to a less extent, the strongest passages having to be sharply limited. A control characteristic according to curve II of Fig. 3 fully answers these requirements so that the invention may very advantageously be used also for this purpose.

I claim:

1, A delayed automatic gain control circuit arrangement for a signal amplifier, comprisin an electron discharge amplifier tube having cathode, grid and anode electrodes to which normal operating potentials are applied, comprising means to apply a signal to the grid of said tube, means to produce a control voltage proportional to the amplitude of said signal above a predetermined amplitude, a source of positive bias potential, means coupled to the anode-cathode path of said tube to produce a second direct potential at amplitudes of the applied signal below said predetermined amplitude, said second potential having a value equal to said positive bias potential, and means to apply said positive bias potential, said second direct potential in opposition thereto and said control voltage between the grid and the cathode of said amplifier tube.

2. A delayed automatic gain control circuit arrangement for a signal amplifier, comprising an electron discharge amplifier tube having cathode, grid and anode electrodes to which normal operating potentials are applied, comprising means to apply a signal to the grid of said tube, means to produce a control voltage proportional to the amplitude of said signal above a. predetermined amplitude, a source of positive bias potential, a

resistor interposed in the anode-cathode path of said tube to derive a second direct potential proportional to the amplitude of said signal at amplitudes of the applied signal below said predetermined amplitude, said second potential having a value equal to said positive bias potential, and means to apply said positive bias potential, said second direct potential in opposition thereto and said control voltage between the grid and the cathode of said amplifier tube.

3. A delayed automatic gain control circuit arrangement for a signal amplifier, comprising an electron discharge amplifier tube having cathode, grid and anode electrodes to which normal operating potentials are applied, comprising means to apply a signal to the grid of said tube, means to produce a control voltage proportional to the amplitude of said signal above a predetermined amplitude, a source or" positive bias potential, a resistor interposed in the anode-cathode path of said tube to produce a second potential at amplitudes of the applied signal below said predetermined amplitude, said second potential having a value equal to the bias potential required for maximum gain of said tube under said normal operating potentials plus said positive bias potential, and means to apply said positive bias potential, said second direct potential in opposition thereto and said control voltage between the grid and the cathode of said amplifier tube.

4. A delayed automatic gain control circuit arrangement for a signal amplifier, comprising an electron discharge amplifier tube having cathode, grid and anode electrodes to which normal operating potentials are applied, comprising means to apply a signal to the grid of said tube, means to produce a control voltage proportional to the amplitude of said signal above an ampli tude at which fading phenomena substantially disappear and having a positive bias component, a resistor interposed in the anode-cathode path 8" of said tube to produce a second direct potential proportional to the amplitude of said signals at amplitudes of the applied signal below said amplitude at which fading phenomena disappear, said second potential having a value equal to the bias potential required for maximum gain of said tube under said normal operating potentials plus the value of said positive bias component, and means to apply said control voltage and said second direct potential in opposition to said positive bias component between the grid and cathode of said amplifier tube.

5. A delayed automatic gain control circuit arrangement .for a superheterodyne radio receiver, comprising an electron discharge mixer tube having cathode, input grid and anode electrodes to which normal operating potentials are applied, comprising means to apply a signal to the input grid of said tube, means to produce a control voltage proportional to the amplitude of said signal above a predetermined amplitude and having a positive bias component, a resistor interposed in the anode-cathode path of said tube to produce a second direct potential at amplitudes of the applied signal below said predetermined amplitude, said second potential having a value equal to the bias potential required for maximum gain of said tube under said normal operating'potentials plus said positive bias potential, and means to apply said positive bias potential, said second direct potential in opposition thereto and said control voltage between the input grid and the cathode of said mixer tube.

HERMANUS JOSEPHUS VAN BEEK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 

